Conventionally, various technologies have been proposed for detecting failure in an inverter circuit, such as failure of a switching element or failure of a current sensor (see, for example, PTL 1 and PTL 2).
Furthermore, conventionally, when a failure in an inverter circuit has been detected, if the failure in the inverter circuit occurs in a single phase, for example, then drive control of the motor can be continued in the remaining two phases, which are normal phases.
Here, if the drive control of the motor is interrupted completely when the failure of the inverter circuit is in one phase, then the motor cannot rotate under its own torque thereafter. In particular, in the case of an electric power steering device, turning the steering wheel by the driver's steering force alone places a large burden on the driver. Therefore, motor drive control is continued in the remaining two phases, which are normal phases.
Furthermore, when continuing the motor drive control, it is necessary to switch from the normal drive control to drive control corresponding to failure after a failure has been detected in the inverter circuit, and therefore a failure of this kind must be detected rapidly. In other words, the greater the delay in the detection of a failure in the inverter circuit, the less possible it becomes to continue drive control of the motor in an appropriate fashion.